Some birds seem straight up freaked out about new toys or perches. My Hahns Macaw, Gizmo, was one of those birds. New items would send her screaming and clinging to the side of the cage to get away. I knew I would have to be very slow with her.
It can be a long process, but I have used this with several birds now with good results.
Choose ONE new item you would like to introduce to your bird. We want to find a place outside of the cage we can put the item, at a distance where they can see it but don't react negatively to it. For Gizmo, this was about 6 feet away on a stand I had. Leave it there for a week or so. Then move it closer to them by 6 to 12 inches, even if this means it is on the floor; again, able to see it but we want no negative response. Wary is okay as long as they aren't freaked out.
Repeat the process until you can actually attach the item to the outside of the cage furthest from their favorite roost (usually where they sleep or spend the most time). If there is a negative reaction, go back a step or two. We want then to be able to see the toy but not feel forced to have it next to them, and they usually feel safest in their favorite place.
Finally, I switch the item to the inside of the cage directly on the other side of where it had been on the outside. That way it's really only moving from outside to inside, not more than a few inches closer.
Once they are okay with that toy (again, wary is okay. We just don't want them anxious or scared) then you can start desensitizing the next item. Don't rush them though. They may not be ready for something new just yet. Go at their pace, always. Don't feel you need to completely fill the cage with toys, and remember that even non-fearful birds may need a few weeks to get around to playing with new items.
Eventually, after they are used to what is in their cage, and are not playing with them anymore, I do what I call "musical chairs." I mentally divide the cage into several zones that I want to be able to place toys, making sure that there is a range between the "Favorite Perch" and the "Desensitizing Area" of the cage. For timid birds, this might only mean 2-5 toy zones in their cage at any one point.
I rotate the toys so that the toy nearest the favorite perch is retired for a bit if it is still useable and not a comfort item for them, or thrown away if they have destroyed it. I move the next closest toy in its position, and the next closest moves into that now empty spot, and so on and so forth, so that the most recently Desensitized toy would move farther into the cage and make room for a new one eventually. In this way, only one new element is added at a time, while the rest are known things and just shifted. It allows them to have the extra time they need with toys without getting bored. For some birds, placement is key, so rotating them through also gives them a chance to tackle a toy in a different position that they may prefer.
For my Hahns, some things were able to be moved in within a couple weeks, while others took a month or more. It's all about slightly pushing that comfort zone in manageable amounts. Luckily, Gizmo did learn to read trust me more through all of this and I was able to swap things in and out of her cage without so much build up after a while. She taught me so much though about patience and working with fearful birds that I will always be grateful for!